I thought this code would do what you say is impossible (I do believe
you, though =P). The problem it runs into is that the call to raise
(produced by the identifier-syntax) isn't necessarily evaluated first
in an offending expression. (And if you raise the exception in the
transformer rather than creating an identifier-syntax, it doesn't get
caught by the with-exception-handler in test.) There may be bigger
problems with it, though - if you feel like educating, let me know if
you see any.
(define-syntax define-missing-exports
(lambda (o)
(syntax-case o ()
((_ var ...)
#`(begin .
#,(let rec ((ls #'(var ...)))
(cond
((null? ls) '())
((call/cc
(lambda (k)
(with-exception-handler
(lambda (x) (k (not (undefined-violation? x))))
(lambda () (eval (syntax->datum (car ls))
(environment '(library-being-tested)))))))
(rec (cdr ls)))
(else
`(,#`(define-syntax #,(car ls)
(identifier-syntax (raise
(make-who-condition '#,(car ls)))))
. ,(rec (cdr ls)))))))))))
(define-missing-exports set of all possible exports from the library
being tested)
(define-syntax test
(syntax-rules ()
((_ expr)
(call/cc
(lambda (k)
(with-exception-handler
(lambda (x)
(if (who-condition? x)
(begin
(display (string-append "missing export: "
(symbol->string (condition-who x))))
(newline)
(k))
(raise-continuable x)))
(lambda () expr)))))))
On Sun, Jun 14, 2009 at 9:25 PM, Abdulaziz Ghuloum<[email protected]> wrote:
>
> On Jun 14, 2009, at 2:10 PM, Ramana Kumar wrote:
>
>>> So, you have a library that may or may not export a macro, say let^,
>>> and you have an expression that may or may not reference let^. If
>>> the expression references let^, and let^ was not exported, which
>>> would normally result in a syntax error, you want to defer the error
>>> until run time instead of being signaled at macro-expansion time.
>>
>> I actually don't want the error at all - I want to detect that let^ is
>> not exported and therefore remove the expression that references it
>> (or replace it with something). This can happen at macro expansion
>> time. Still not possible?
>
> It's the same thing. You want to handle the syntax-error exception by
> replacing the offending expression with another one of your choice.
> The replacement may be just #f or (asserion-violation 'foo "unbound")
> or whatever. Still, not possible to do correctly without hacking on
> psyntax.
>
> Aziz,,,
>
>
